text:
What privileges do off-duty pilots have in their own airline?
I was recently in a Delta flight where there were two off-duty, but in-uniform pilots flying along with the passengers. One of the pilots sat in the First Class section of the airplane and the other sat in the Economy section. After the seatbelt sign came off, the pilot in the Economy section moved up to the First Class where there was apparently an empty seat.
This led me to wonder: what privileges do off-duty pilots have in-flight?
I am limiting this to only US to keep it from being "Too Broad".

If they were in the back in uniform, they may not have been off-duty, but simply deadheading.  This is assigned travel to get into position for later flying (either that day or the next), or back to base after they finished flying someplace else.  That's still on-duty, but just riding in the back.  There are other alternatives as well, but deadheading is an obvious possibility there.

@RalphJ True, but I was meaning "off-duty" as in "not flying the plane".

Does "not flying the plane" inclde another set of crew required on flight longer than 8 hours?

@vasin1987 It was only a 2 hour domestic flight.

@vasin1987, the relief crew on long flights is accommodated in crew rest area, which long-range aircraft have, not in the passenger cabin.

This is something that will vary with each airline so it's still pretty broad.

This is airline specific, so there's no single true answer: but I'll give an overview of what's "normal practice" for many airlines:
Airline staff will typically be bumped to First/Business class when there are available seats, although they're not usually top of the list.
In a typical airline, frequent flyers will be bumped first (particularly those with a lot of business class miles but who happen to be travelling economy right now): it's a way of rewarding loyal/frequent customers.
Once the frequent flyers are all moved up, though, the seats will often be filled with staff or contractors. Typically deadheading/on duty staff first (ie those who are flying home from a previous flight that they were working on), followed by off duty staff and their families.
This is airline specific, however - some airlines will bump everyone on the frequent flyer program  until Business is full, leaving no space for staff. Others will not move any passengers up (to discourage business class flyers deliberately trying to "game" the system by booking economy on quiet flights knowing that they'll likely be moved up), leaving more space for staff. Some airlines will not bump staff at all, or will only do so for deadheading staff (those who are flying because of work, not for their own holiday etc).
There's a lot of variation on things like whether your family will be bumped with you, or whether only higher-level contractors will be moved up. I used to work for a company which worked with airlines, and typically our directors would be moved up to Business while the lackeys were left in Economy


text:
Do aircraft manufacturers like Airbus or Boeing charge the airline, when they assist in aircraft accident investigations?
Most of the time, when an aircraft crashes or has an accident, the manufacturer also assists in the investigation. Is this a pro-bono kind of effort on part of the manufacturers, or do they charge the airline for it?

It is usually in the best interests of the manufacturer to be involved in any investigation of an accident that is significant enough to have some type of impact on the manufacturer from a liability, regulatory or public relations point of view. I doubt the manufacturer would want to be compensated for its efforts, but you never know. Sometimes the manufacturer gets involved because they are asked to do so.

@JuanJimenez Also depends to the extent of involvement of the manufacturer.

At Airbus, there are investigation leaders involved once or twice a year in major investigations (10 investigations a year for each one). They can be supported by a "go team" of 70 experts, trained and equipped to face a difficult environment, who can be deployed within 24 hours. Other R&D experts can be solicited. So your question is interesting, not only for the investigation phase, but also for the funding of this permanent pool of experts (source, in Fr).

Investigating agencies are in charge of most air accident investigations not involving criminal acts. In the US this is the NTSB, which is required to investigate air accidents per US Code Title 49, Chapter 11. These agencies exist to provide independent investigation of accidents and provide recommendations to regulators.
The investigating agents will do their best to understand the circumstances and causes of an accident. Agents of the NTSB have authority under §1134 of the above mentioned law to gather information about an accident.

(a) Entry and Inspection.—An officer or employee of the National Transportation Safety Board—
  (1) on display of appropriate credentials and written notice of inspection authority, may enter property where a transportation accident has occurred or wreckage from the accident is located and do anything necessary to conduct an investigation; and
  (2) during reasonable hours, may inspect any record, process, control, or facility related to an accident investigation under this chapter.

Investigations can also benefit from working with people that are already familiar with the critical elements. As part of the investigation, the agency may invite various parties. This could include airlines, manufacturers, air traffic control, workers unions, other agencies, or anyone else with information or expertise that could be helpful. If these parties do not abide by terms for participation, this status can be revoked. The resources spent by parties during an investigation can in some cases be fairly intensive. For example, while supporting the investigation of the crash of British Airways Flight 38, Rolls-Royce and Boeing ran full-scale tests of the engine and the fuel system designs.
Investigating agencies are funded by their governments. The Aviation Safety department of the NTSB has an annual budget of about $50 million. Parties to the investigation represent their respective parties and are paid by their party to do so. Part of the party agreement acknowledges this:

Participation as a party to an NTSB investigation is a privilege and confers no rights or benefits.

They "pay" that parties get is good press for assisting in the investigation, and helping to both prove and improve the safety of their products and services.

While that looks to cover the US side, the question seems to ask about Airbus as well, which is a French/German/UK/Spanish company. (RR or Bombardier would be UK and Canadian companies respectively ...) While the question may be a bit too broad in that case, do you have any information regarding other rules and regulations related to this?

@KorvinStarmast I think the lead agency matters more than the nationality of the parties involved, and that the system would be fairly similar in other countries. I'll see if I can find more information though.

Manufacturers are not necessarily involved in the investigations of an airplane crash while they may lend a hand they have no jurisdiction to cary out an investigation (generally speaking).
This depends on where the accident occurs as well as the aircraft involved. Aircraft Accident investigation is generally the responsibility of the nation and national organization that belongs to it, in which the accident occurs. However in practice not all nations have the facilities or resources to carry out an investigation completely so there is a lot of coordination. The NTSB will generally lend a hand if an American aircraft built or registered is involved or help is requested for something like a blackbox read.

If an accident or serious incident occurs in a foreign state involving
  a civil aircraft of U.S. Registry, a U.S. operator, or an aircraft of
  U.S. design or U.S. manufacture, where the foreign state is a signator
  to the ICAO Convention, that state is responsible for the
  investigation. In accord with the ICAO Annex 13 SARPS, upon receipt of
  a formal notification of the accident or serious incident that may
  involve significant issues, the NTSB may designate a U.S. Accredited
  Representative and appoint advisors to carry out the Obligations,
  receive the Entitlements, provide Consultation, and receive Safety
  Recommendations from the state of occurrence.

These agencies are government run/funded. The agencies can call on the manufacturer for advice/help such as requesting information on the aircraft or design. Although much of this data is submitted during the type certification process and already on hand.
Often times the manufacturer will offer to aid if at all possible. One thing to consider here is that his can be a conflict of interest. Most countries treat accident investigations as an open crime scene. Since the cause could potentially be a design flaw it is in the best interest of the investigating body to keep makers out of the investigation so as not to sway it.
Boeing does how ever have a team of investigators (that it presumably pays internally) that they send to accidents involving Boeing aircraft.

International protocol, defined by the International Civil Aviation
  Organization treaty, establishes that a government with jurisdiction
  over an accident site leads an investigation. Accredited
  representatives from the countries of the airplane’s manufacturer,
  designer, operator and registry are invited to join the investigative
  team. For international accidents involving Boeing airplanes, the
  accredited representative is the U.S. National Transportation Safety
  Board. The NTSB leads all domestic investigations. Boeing’s role is to
  support the NTSB as technical adviser.

In a legal sense the NTSB is actually Boeings seat at the international table when it comes to Investigations. While the NTSB can (and often might) call on Boeing directly they are ultimately responsible for the investigations as well as their cost.

Your last sentence looks like it addresses the core of the question.  I think the question is asking for a bit more meat on that element, and the point you cited in the quotation above that.   I have a PoC who (I think) used to participate in that role at Boeing, I'll email him and see if he has more.

@KorvinStarmast your buddy may be able to shed some light but my understanding is there is not a whole lot of meat there as the manufacturer is somewhat uninvolved. Investigations are run/staffed/carried out by almost entirely governmental agencies so they can be impartial. The rest of the answer is intended to emphasize that manufacturers are quite uninvolved and there is little to no info on payment as a result.

Dave, I heard back from him.  His recall was that support to NTSB was subsumed in Boeing's overhead in the budget under a specific category of overhead for safety, some of which was stuff like investigation support and some of it other safety issues.  He recalls no reimbursements from the NTSB during the investigations he was involved with.   As to "uninvolved" they certainly don't lead, but IIRC from AF 447 Airbus was involved in quite a bit of detailed support to the BEA for that accident.

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text:
Is a magnetic compass required for an IFR flight?
Is a stand-alone magnetic compass required for an IFR fight when the aircraft has a glass panel with a Horizontal Situation Indicator?

possible duplicate of Is a magnetic compass required for VFR?

@rbp Um, not to be rude but...  Unless IFR and VFR became the same thing, these are not the same question.

@Jay rbp is correct, the answer to the other question anwers this question though, as the text further down states that all items required for VFR flights are also required for IFR flights.

@SentryRaven Ah, fair enough.  I guess I'm more looking at it from a "search" perspective.  People looking for an answer to an IFR question aren't likely to look at a VFR question, that's all I'm saying.

I think it's worthwhile leaving this as a standalone question: Although the other includes mention of IFR flight as part of the context of the answer, it is still a better fit for the SE Q&A format to keep this as a separate question, unless we also modify the Question of the other post to better fit a generic IFR/VFR answer.

A magnetic heading indicator is required, but not necessarily a wet compass, like this one:
Image Source: Wikipedia - Author: Chopper
Digital devices or combined devices are also possible:
Image Source: WikiPedia - Author: Mysid
The required equipment for VFR flights states, as already posted in this question:

§91.205   Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements.
(a) General. Except as provided in paragraphs (c)(3) and (e) of this section, no person may operate a powered civil aircraft with a standard category U.S. airworthiness certificate in any operation described in paragraphs (b) through (f) of this section unless that aircraft contains the instruments and equipment specified in those paragraphs (or FAA-approved equivalents) for that type of operation, and those instruments and items of equipment are in operable condition.
(b) Visual-flight rules (day). For VFR flight during the day, the following instruments and equipment are required:
...
(3) Magnetic direction indicator.
...

The text further states for IFR:

(d) Instrument flight rules. For IFR flight, the following instruments and equipment are required:
(1) Instruments and equipment specified in paragraph (b) of this section, and, for night flight, instruments and equipment specified in paragraph (c) of this section.
...

Note that as @rbp points out the magnetic direction indicator is not required to be a separate piece of hardware (such as a wet compass). My understanding is that for glass panels it's usually integrated into the HSI by slaving that to a magnetometer (which has the benefit of eliminating the need to correct the DG/HSI for gyroscopic precession as well).

@voretaq7 I edited my answer to incorporate that.

The device depicted is not a magnetic direction indicator. It is a HSI containing a heading indictor which works on gyroscopes.

@PaulOgilvie, but it has no correction knob, so it must be slaved to a magnetic device. A heading indicator would be this:
http://en.wikipedia.org/wiki/File:C172_heading_indicator.jpg

@SentryRaven, how would the deviation be set? (Difference between magnetic and true North.) The pilot will want the true North heading to be displayed on the device in front of him.

@PaulOgilvie You mean the variation? Deviation is usually read on magnetic compasses via a deviation table next to the compass. I am not aware of variation ever being entered into HSI or Compasses, you factor it in during flight-planning.

@SentryRaven, when you set the heading indicator to the compass, you must then still correct for the variation (indeed, "variation", not "deviation" - getting sloppy on my terminology) so the heading indicator points to true North, not magnetic. This difference can be very significant (when flying to Grand Canyon it was 15 degrees or so!). You also have to regularly adjust the heading indicator as the gyroscopes keep it in its originally set position while true North "moves" as you airplane "moves". The HSI is to my knowledge a heading indicator, not a compass. (I am a C172 pilot.)

This must be a difference in training, I never set my gyro to true north, but slave it to the readout on my wet compass. Variation is only compensated during flight-planning, not on the device itself via adjusting. In the case of the above HSI, there is no adjustment knob to correct for variation or to re-adjust due to gyro-drift, it must be slaved to a magnetic device, as the heading select and course select knobs are for different things.

@SentryRaven, you are right. I haven't flown in a couple of years. Must have lost a lot. The heading indicator is set to the compass; variance is applied to the course on the map (which is drawn in true North) during flight planning, on the ground, and in the cockpit when you have to ammend your plan.

If you mean a "whiskey compass," like the thing that sits above the center of the panel, then the answer is no.

For aircraft certified under Part 23, as described in AC 23.1311-1C:

Section 23.1303(c), Amendment 23-62, amended the requirement from “A
  direction indicator (non-stabilized magnetic compass)” to “A magnetic
  direction indicator.” As new technology becomes more affordable for
  part 23 airplanes, many electronic flight instrument systems will use
  magnetically stabilized direction indicators (or electric compass
  systems) to measure and indicate the airplane heading to provide
  better performance.

A fluxgate compass, which can be remotely mounted in the wing or the tail would also satisfy the regs.
Also, MEMS fluxgates are being developed right now, which could be integrated directly into an AHRS. And there are a number of other technologies being developed to sense the earth's magnetic field.

I do wonder though, what do they do in case of an electrical failure? No mechanical backup required?

This still effectively requires a magnetic compass ("magnetic direction indicator") - a device which uses magnetism to determine your current heading, it just doesn't have to be a traditional wet compass. You could not, for example, legally fly a glass panel aircraft which only had GPS and MEMS or laser gyros in order to determine and display heading.

@voretaq7 but you could with a solid-state fluxgate compass

Yes - a fluxgate (or other electric magnetometer) is still a "magnetic direction indicator" :)

Does the definition "magnetic direction indicator" allow the pick-up device to be separated from the display device? Or is one of the aspects of the requirement that, when everything else fails, there is still a reliable (magnetic) heading indicator available in the cockpit?

It's actually better to have the sensor on the wing or in the tail to reduce electro-magnetic interference.

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text:
If turning off electronic gadgets is so important, why are there no detectors used to find them?
It's supposed to be that electronic gadgets' emission are dangerous for airplane systems. If this is true, why don't they use any detectors to locate such emissions?

This is an interesting point.  Where I work they actually have devices to find hotspots that might be broadcasting our intranet.  They're pretty accurate, and quite cheap.  Not sure why airlines don't use them.

They do, but the detectors are electronic devices and must be turned off at that time :)

Note that personal electronics are no longer banned by the FAA, and many airlines are also changing their policy to allow them at any point in the flight. The FCC still has regulations against using cellular signals from the air, although the chances of getting a signal from cruising altitude is roughly zero anyway.

@JayCarr Where I work, we design and build devices for detecting transmitters. While the ones for detecting only wi-fi are cheap, the ones for detecting any kind of transmitter are not cheap at all, especially if they're good at picking out a single signal. In order to do that, you need a full-fledged spectrum analyzer.

Personal electronic devices are now allowed in all flight phases, as Bret's link points out. Only cellular radios are banned because they can interfere with the ground cellular network. Even there, the FCC is considering lifting that ban, though the Department of Transportation is considering implementing it's own cellphone ban because the vast majority of passengers don't want to hear the guy next to them yapping on the phone during their flight.

Regarding the cell phone ban, Delta Airlines has the following to say for the hypothetical scenario where the government allows use of cell phones on board: "We have years of customer feedback on in-flight voice communications. The overwhelming sentiment is to continue to not allow voice communications while in flight." Delta FAQs - PEDs

@reirab: If you need a super complex device to intentionally pick up the signal, how comes the airplane itself accidentally picks up that signal and then gets influenced by it?

@MSalters Ensuring that you will be able to pick up the signal and identify it vs. having an outside chance of being interfered with by it are two very different things... Also, when devices are functioning properly, they should not interfere with aircraft systems. Apparently, the FAA now thinks that the chances of a PED interfering are small enough that they lifted that ban on PED use during critical flight phases. Only cell phone use is banned now and that's by the FCC (for potential interference with the cell network,) not by the FAA.

Electronics don't interfere with anything in the aircraft

Three reasons I can think of:

It's easy to detect the general presence of a specific transmitting device (a WiFi antenna, say), but very hard to precisely localize it. They could tell if there's one on the plane, but someone with a very sensitive piece of equipment would have to walk up and down the aisles pointing it at people to actually find it in somebody's pocket. And cellphones (when not in use) are generally burst transmitters: they don't broadcast continuously, they just "ping" the tower every so often. This means it could take a long time and a lot of aisle-pacing.
Many "electronic devices" which are banned are entirely passive and have no traceable[*] emissions. Think cameras, handheld GPS receivers, iPods. (On the other hand this passiveness also means they are extremely unlikely to harmfully interfere with the operation of the airplane, but right or wrong, such devices are still banned in some cases.)
It's not that important. There are a few incidents where electronic interference is suspected as a possible factor, but no conclusive evidence. As soon as there is actually a fatal accident where an electronic device can be conclusively blamed as the cause (and I doubt this will happen), the airlines will be a lot more willing to spend the kind of money that would be required to implement this.

For a full, evidence-based discussion of the actual risks, I refer you to this question on Skeptics Stack Exchange: Are personal electronics a risk to commercial aviation?
[*] No traceable emissions. All electronic devices generate some very, very tiny incidental signals. But these are so weak that they will be virtually impossible to detect on a crowded plane filled with its own (approved) electronics. Such emissions are measured in laboratories with extremely sensitive antennas in special, heavily-shielded rooms under carefully controlled conditions.

just don't let the plain to take off until the gadget is turned off 2. Why they are banned, then?

@user626528 There's always going to be that one guy who refuses to turn his phone off (or isn't aware that it's on). Are you going to cancel the flight for this? What if someone leaves a phone turned on in their checked luggage? Pull all the bags off the plane and search them? What if it's one of the ground crew? As for why they're banned, there are a lot of reasons. FCC bans them to reduce the chance of disrupting the cell network on the ground. Flight attendants want them banned so that passengers will pay attention and not be obnoxious. This could make a full independent question.

@user626528 There are also some "Related" questions over on the right-hand side that discuss more about "why they're banned."

If turning off electronic devices has any importance - yes, it's better to cancel the flight. And if it's not important at all - well, just stop this stupid cargo cult theater.

@user626528 On that, I can only encourage you to check out the Skeptics link. The topic is very thoroughly covered there. But I agree with your last sentence completely. Things are (slowly) moving in that direction with airlines gradually relaxing their rules.

Many of the devices you mention (camera, iPod, GPS) aren't actually banned in the US. But it also may depend on your specific airline (and national) rules.  Then again, it also comes down to obeying flight attendant instructions (story of guy told to "turn off" his film camera that doesn't use batteries).
Personally, I've had a small plane lose GPS on final due to interference, and while operating VFR, that's no big deal.  But if there's a chance it could be more of an issue? Not worth the risk.

@user626528: you can think of the situation (before the recent lifting of restrictions) as this: if the occasional flight has a device or three left on, then there is an absolutely minuscule (tolerable) risk of a consequent accident. However, if every flight had dozens or hundreds of devices switched on, then there is a small (intolerably high) risk of a consequent accident. So asking passengers to turn off devices was sufficient since almost all co-operate. As evidence accumulated, the latter small risk has been re-assessed as minuscule.

Due to plentiful aircraft shielding and low power emissions of personal electronic devices, practically speaking only a malfunctioning electronic unit that is transmitting the exact same frequencies that the antennas are designed to receive and filter out carry significant probabilities of affecting avionics. One hundred iphones with wi-fi turned on would almost always have an effect, as they would not be emmiting signals received by the antennas. (If there was a rare malfunction a signal could pass from the cabin, through the window and into a GPS receiver, for example).
To identify the position of a malfunctioning unit one would need expensive multiple receivers and a computer that triangulates the location.  This is just too expensive for the low risk.
Even if you could identify a malfunctioning unit, the unit would notmyet be malfunctioning otherwise the aircarft would already be receiving the bad signal and in a sense the aircaft systems are the first detection.
On the whole the answer lies in the fact that the interference for the odd electronic, people forget to turn off is low risk and does not justify the expense of an advanced locator system.  It is the aircraft that provides the saftey by properly shielding its electronics from interfering signals to begin with.  The antennas restrict communication on narrow frequencies amd usually with certain communication protocols, so even at the right frequency disturbing the aircarft instrumentation or navigation is a freak accident. I think the most probable interference is that a malfunctioning noise transmission across various uncontrolled frequencies could distort the coding a proper signal trying to communicate through the antenna to a given receiver.

@dvnrrs - yes I understand that but in practical terms I do bot think the aircraft has much to worry about because of the shielding, except those frequency that actually pass through intended filters.  I am speaking more from experience as I have written EMI/EMC tests for avionics installations. However I could be wrong as I have only heard second hand information of occurrences actually taken place in flight through this means.Anyway I should edit the first line to make it clearer.

OK, I removed my downvote. However don't take this personally but I would still refer the interested reader to the thread on Skeptics which is thorough and heavily sourced using primary material. They don't have to "take either of our words for it."

@dvnrrs - interesting link and worth the read. thanks.

The shielding from the aircraft itself can't protect against signals that are broadcasting on the same frequencies used by the avionics (i.e. signals like those you describe in your first sentence.) In order to do that, they'd block the signals they're trying to receive, too.

One thing to point out, electronics on planes really isn't as big of a safety factor as the airline would have you believe, and it's becoming less and less of an issue since consumer demands are pointing towards the aircraft that allow them to be entertained while traveling, such as have WiFi.
For the companies that run the phone lines it's a different story entirely. If your careening through the air at 500 knots and you make a phone call, you require the phone towers to do a lot more work receiving your call. I'm not sure why, but I remember reading that the main movement behind not using electronics while flying was really because our telephone infrastructure isn't capable of handling the excess load.

You have some good points with the cellular network having difficulties: one is your effective range at an elevation of 30,000ft MSL (hint: quite good!), and the other is, you're moving so fast even if you did ping a tower, you'll be moving into a new cell anyway.  Having it on wouldn't do a bit of good for you, so might as well use airplane mode. :)

This applies only to cell phones. The reason for turning off all electronic devices in critical flight phases was indeed interference with the aircraft. At least in the U.S., that ban has been lifted. The ban on cell phones remains in the U.S. and is banned by the FCC due the fact that it used to wreak havoc on the cell network (in range of too many towers at once, handing off extremely frequently, etc.)

The greatest danger appears to be when an aircraft is flying slowly, with landing gear open  and there are cell towers on hills level with the approach path, because these are circumstances when a cell phone is most likely to make a call connection.
Many years ago working as an airline baggage handler in a country which prohibited all cell phone use during commercial flights, I was astonished to hear baggage carts with phones ringing all the time and it is the airport baggage halls where one could implement this policy or at the check-in counters. I imagine airlines would not be too keen on the extra manpower and wages required.

The reason PEDs used to be banned during critical flight phases (taxi, takeoff, and landing) wasn't because that's when a phone is most likely to complete a connection, but because that's when accurate readings on the radionavigation equipment is most critical (especially final approach in low visibility conditions.)


text:
What is the official EASA definition of A, B, C and D checks, or where do I find it?
Schedule Maintenance is divided into many different checks. What is the official definition (EASA website, not wikipedia) of A, B, C and D check, or where do I find it?

I edited the question slightly to focus more on what the definition actually is, and less on where to find it. Feel free to [Edit] further, or to roll back, if you disagree.

Seems like no official definition. Just a grouping of required maintenance items put together in a schedule that change with airlines.

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text:
Why do some commercial flights start off unpressurized?
In this video, the B738 starts takes off without the plane's pressurization system engaged.  The video specifically notes that the craft is being pressurized fairly soon after the takeoff has happened.
Is this because of the icing conditions they had on the ground?  Or is there another reason why it would take off unpressurized?

Screen capture for the extremely busy:

Was WAI (wing anti ice) selected? Then I would expect that the pilot wanted to use all available bleed air for heating the leading edge slats right after take-off, avoiding the additional strain of pressurizing the hull.

@PeterKämpf  On the 737, the wing anti-ice switch snaps to the off position when the thrust levers are advanced for takeoff (it's solenoid-held on, spring-loaded to off).  So wing anti-ice is always off for the takeoff roll itself by the design of the system.  Once airborne you can turn it on. At which point, though, you're clean (Type 4 deicing fluid), and I can't think of a scenario where the extra bleed air during the early climbout (until the packs are turned on) matters that much or would show up in performance charts.

On reflection, the performance charts are Bleeds On / Bleeds Off (and off = neither packs nor wing anti-ice getting engine bleed air), and Wing A/I On or Off.  There's no option in any charts I've seen for Engine Bleed On, Wing A/I On, Packs Off.  Because the assumption would be that you always pressurize the cabin, rather than shedding that for performance (APU inop + engine bleed off for T/O being the sole exception).  Inop APU with a Bleeds Off takeoff driven by performance considerations (runway braking action) seems like the only plausible scenario, IMHO.

The 737 like most modern airliners (notable exception, the 787) is pressurized by bleed air, from either the engines or the APU. Drawing bleed air from the engines reduces the thrust available for takeoff.
From most common to least common, takeoff configurations are:

Engine bleeds on (supplying bleed air to the pneumatic system), APU off. This is a pressurized takeoff & generally the most common scenario.
Engine bleeds off, APU bleed supplying air to the pneumatic system. This also results in a pressurized takeoff, with more engine thrust available. Used when max thrust is desired, typically due to heavy weights, short and/or contaminated runways, close-in obstacles requiring steep climb gradients, and similar circumstances.

In the case that the APU is inoperative but taking off with the engine bleeds off is still required, you get:

Engine bleeds off, APU bleed off. This results in the unpressurized takeoff referenced in the question.  At a suitable altitude (typically 2000 to 3000 feet into the climb, a minute or two after takeoff), the engine bleeds are turned back on & the aircraft pressurizes normally.

This is uncommon, but still considered a normal procedure (not an abnormal one out of the QRH).
Edit:
Link to Pneumatic System description for the 737
Link to PDF discussing pneumatics & pressurization on the 737

This seems to contradict KeithS' answer.  Do you have some citations for what you are saying?  Keep in mind the aircraft in question is a brand new 737-800 (which may or may not make a difference I guess.)

@RalphJ: Climbing is not so bad for ears. Even divers, who experience much larger pressure differentials, have to swallow or blow their ears when diving, but surfacing is rarely a problem. I do agree that the cabin starts pressurizing immediately though. In fact, it usually starts pressurizing even before takeoff.

Maybe the pilot wanted to have more bleed reserves to support the wing anti ice system, which runs on bleed air from the engines and is activated after take-off by a weight-on-wheel switch if it has been selected on by the pilot.

@PeterKämpf That would make a lot of sense.  I didn't hear anything along those lines from the video, but I wouldn't know what I'm listening for.  I don't doubt it though, considering there were ground icing conditions (they do say that.)

@JanHudec  Agree that unpressurized rapid descents are WAY worse for the ears than a climb, but it'd still be clearly noticeable.  Normal cabin rate is maybe 250'/minute; it might not damage any ear drums, but 3000'/min rate would be a lot less pleasant.

The air pressure outside decreases as the aircraft climbs. However up until a certain altitude, which I happen to forget, you can fly without the aircraft being pressurized. This is why we don't need to throw on oxygen masks while flying light aircraft (up until a certain altitude anyways)

I'm pretty sure that in most cases commercial flights take off with pressurization, that's why this flight had pressurization after takeoff as a special procedure.  I'm wondering why they decided to use the special procedure instead of the normal one.  All the same, you are correct (in the general sense), and welcome to the site :).

I'm afraid you omit an important thing: The fact that the aircraft is at the correct pressure is different from the fact whether the pressurization switch is on or off.

@yo' Excellent point, modified the question so it's a bit clearer.

I'm sorry @yo' -- what exactly is this "Pressurization Switch" you're referring to?  I'm pretty familiar with the 737, and "the pressurization switch" is a new one on me.

This is all correct but it doesn't answer the question, which is why a commercial flight might take off unpressurized, not why doing so is safe.

@RalphJ I can't find a free photo, but if I'm not mistaken, it's the bottom right switch at the first photo in http://www.b737.org.uk/pressurisation.htm with three positions: AUTO, ALTN, MAN

@yo'  The switch you refer to is the pressurization mode select, which tells the controller what mode to operate in.  But the controller is essentially always going to pressurize the aircraft, or at least try to (it only controls the outflow valve), while in flight. Whether you pressurize or not is determined by the supply of bleed air (engines + APU) and if the packs are switched on or off. Point being, there is no single "Pressurization-ON" or "OFF" switch in a 737. There are switches for each bleed + pack, & for the pressurization controller mode (always "AUTO" unless something has failed).


text:
How does one become an airline pilot in Australia?
I am 19 years old and from a young age I have always wanted to become a pilot at an airline. Becoming one is my biggest dream, to me an airline pilot is the most coolest jobs - I get goosebumps just by thinking about me in that position.
I have finished my high school education and hope to go to Australia to study in order to become a pilot. However, most of the university programs don't give me the feeling that I can become a pilot. Most of what is learned and practiced is in small planes, even the career opportunities don't mention that one will become an airline pilot.
Does anyone know the best and fastest way to make my dream come true? What should I do?

The only course that has got me attracted to is the one that's in Swinburne University, but then again, small planes. I am fine with flying small planes, but where does that big jump to airplanes happen, if it's not taught in university. Where do I learn it? Does anyone know the procedure or the best place to train to become a pilot?

Also, I'm not an Australian so I will be an international student. I have checked for jobs in my home country airlines, they require a minimum of 5000 hours of flight, does this mean normal plane flight hours? Where will I get these hours covered from? Of course this many hours aren't offered in university.

You may want to do a couple things to help us help you.  Firstly, try and keep your questions a bit more general (ie., how does any person become an airline pilot in Australia.)  Secondly, you are asking several questions in the same post, why not break this post up into several questions so they can be treated individually.  We'd love to help you out, it's just hard to answer your questions when they are posted in this fashion...

And another thing: use some more whitespace. Linebreaks can add greatly to readability. And that goes for posts here as well as for job application letters :)

@JayCarr Done editing the title

Okay :) I will. But, the description is all related to becoming a pilot, how to break it up into different questions. The description is what I think about this.

@JayCarr I removed a question that can be asked separately which was "What is the realistic age..." , I dont think the rest of th question can be separated

@jwenting , I made it more readable. Is this better? Is there any way I could improve this? Please let me know

That looks fine, I just hope we have enough people here from Australia to answer the question...

I am Australian and I hope I can help.
Firstly, you don't need to fly through a university. There are hundreds of private flight schools here which are of great quality, and very few of them are associated with a university. More and more international airlines are sending cadet pilots to our schools. Most of them offer theory courses in addition to the flying, or you can always self study.
Having said that, the university courses are great and airlines like people with a degree (but it's not mandatory to have one). You'll be paying more though, it's a trade-off.
Your comment about small planes - All flying is initially in light aircraft - even if you got an airline cadetship you start small. No single course in Australia or anywhere will let your first flight be in a jet.
Speaking of airline cadetships - these are definitely the best and quickest way to get into an airline cockpit but the positions are highly competitive (and maybe not open to international students, I don't know). But you can still get an airline job without a cadetship...
The normal progression is this - you can technically get a flying job after 150-200 hours of private training and flying, having achieved your commercial pilot licence (CPL). I say 'technically' because if you only have minimum hours and minimum qualifications you have to look very hard for a job. Most people straight away get an instructor rating, so they can get a job teaching people to fly. Other early jobs include sightseeing flights, parachute flights, and mail flights to remote towns.
Your training will have been in light aircraft, but fortunately Australia has possibly the largest light aircraft industry in the world, so there's lots of jobs in these planes. It's highly unlikely that your first job will be in anything bigger than a 6 seater plane.
If you get extra qualifications (such as multi-engine endorsement, and instrument rating), you can get more jobs, and gradually fly bigger aircraft.
After you have your CPL, the next big step is the Air Transport Pilot Licence (ATPL). Regional airlines here generally require as a minimum, 1000 flying hours in light aircraft, the multi engine rating, the instrument rating, and passes in the ATPL theory exams - note that you don't necessarily need the ATPL licence itself. It will take most people a couple of years until they get this experience.
If you have that much experience flying 6 to 8 seat aircraft, it's not actually a huge leap to the 20 seat planes of the regional airlines, and the airlines will train you for this step. And once you've flown those for long enough any airline can train you to their jet.
Of course if you want to go back home after the training it will be different. But no school here can train anyone to go straight to an airline. You've got to do the small things first. Think of it as character building!
I'm sorry but I'm a bit rushed for time but I hope I've helped a bit. Australia is one of the best places in the world to train in my opinion (but I might be biased!). Please ask if you have any more questions. And good luck.

How old do will I be when I first start flying an airline? An estimate? :)

It's tough to say. It depends on how quickly you can do your training, how long the training takes (university training in Australia will take longer than just doing it privately), and of course it depends on the airline, if they're expanding they hire more pilots but if they shrink..... If you start training at age 20 I think a realistic time to be applying for a major airline would be 26-27. But it depends. And they normally hire up to about age 40.

I'm not super sure about Australia, but in general terms, I know that it can take a lot of time to get from a regular license to being a commercial jet pilot.
The first step is to get your private pilots license.  So I'd find a program to get that taken care of.  Then you're going to want to look for programs that get you a commercial rating (which simply means you'd have a license that allowed you to make money by flying a plane.)
There is no real big jump at any point, you go from very small planes to progressively larger planes over a period of years.  In fact, when you first start flying commercial, it is likely that you will be flying a somewhat small aircraft for minor tasks.  Like a Beechcraft King Air or something similar.  From there you just build up more and more hours and get jobs with bigger and bigger carriers until one day you land that dream job of yours, flying 747s ;).
Some colleges will help you scoot the process along by getting you a bunch of hours in the smaller commercial aircraft (like the above mentioned King Air.)  But I don't know of any that will have you flying jet airplanes for a larger carrier the moment you graduate.
That being said, I'm not super familiar with colleges in Australia, so perhaps they are significantly different than colleges in the US (which is where I'm from.)

So the path that I am taking is correct? So there is nothing to study after getting the commercial rating? After getting the commercial rating when I apply to flying small planes, how do I apply to airlines? Because there is a minimum of 5000 hours that you need to do, to be able to apply

@Tharindu lol, zillions of questions! What you really need to do Tharindu, is find a commercial pilot you can speak with in person and have a very long conversation with them.  They will probably be able to set you on the right course.  On this stack it's hard to move much beyond generalizations, but a real world pilot can tell you (or an advisor at the colleges you're looking at.)  I enjoy your enthusiasm though!

@Tharindu Or, you might trying joining the chat for this stack exchange, there are a few commercial pilots who show up there and could probably help as well :).  http://chat.stackexchange.com/rooms/12036/the-hangar

Ok :) I will :) Thank you.

I am pilot and currently work as a flight instructor in Australia
In order to become an airline pilot you need to get your initial licenses in light aircraft. No one learns to fly in a big jet. There are two main pathways to get a job in an airline
Self funded and/or university training option: You start training at a flight school to obtain your commercial Pilots licence. Once this is done you hunt for your first job but you are limited to:

Small aircraft jobs (dropping mail cargo sight seeing flights etc)
Parachute plane pilot
Working for a farm station (fence inspection etc)

Typically these jobs are found in remote Australia for a better chance of obtaining a job and if you have drive to do it go to a flight school get an instructor rating and teach people how to fly.
Once you’ve obtain the magical 2500hrs and manage to get enough funding to get your instrument rating (another story) you can then get into an airline as a first officer.
The second option is through a Cadetship program. These programs are highly competitive and can be quite expensive but once completed you get a gig as a second officer. Second officers typically don’t do a lot of flying they’re on the flight deck to assist and monitor the aircraft in cruise after many years monitoring you can get upgraded to first officer where you can finally do some good flying.
The training for both these pathways are the same at a flight school getting your initial licenses on small aircraft
Yourself being an international student it would be very difficult to get a job flying in Australia with the visa requirements the best way to do it if you are unable to get a Cadetship program would be to get your instructor rating but do it at a flight school who could use your skills and could potentially offer employment afterwards. (I have friends that have done this) obviously schools will not say they will hire you but if you prove to them how well you can work anything is possible.
Hope this helps and good luck.

Welcome to Av.SE!

Basically, the same way you become a commercial pilot in the US, or anywhere in Europe, or probably anywhere in the world. Start small and work your way up. Nobody jumps straight to the top in any endeavor.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What certifications must one have to repair/alter propellers and instruments?
According to 14 CFR 65 Subpart D

A certificated mechanic may perform or supervise the maintenance, preventive maintenance or alteration of an aircraft or appliance, or a part thereof, for which he is rated (but excluding major repairs to, and major alterations of, propellers, and any repair to, or alteration of, instruments), and may perform additional duties in accordance with §§65.85, 65.87, and 65.95.

(Emphasis mine)
What certifications are necessary to affect the repairs or alterations specifically excluded from a "mechanic" certificate?

I suspect a Full A&P rating is needed, with a further rating to work on instruments - not that much could be done outside of send an instrument back to the OEM.

I believe you need to be an FAA Certified Repair Station approved to work on instrumentation, propellors etc. as noted here

The FAA lists and approves ratings on a form called operations
  specifications. The operating specifications will also be displayed in
  the maintenance facility, usually right next to the certificate. Like
  other businesses, most maintenance facilities advertise on the Web and
  in trade magazines, so you have many ways to research what rating a
  particular repair station may have. There are basically six ratings
  that pertain to a repair station:
•  Airframe
•  Powerplant
•  Propeller
•  Radio
•  Instrument
• Accessory

To be an approved Repair Station the employees must conform to

§ 145.157 Personnel authorized to approve an article for return to
  service.
(a) A certificated repair station located inside the United States
  must ensure each person authorized to approve an article for return to
  service under the repair station certificate and operations
  specifications is certificated under part 65. (b) A certificated
  repair station located outside the United States must ensure each
  person authorized to approve an article for return to service under
  the repair station certificate and operations specifications is—
(1) Trained in or has 18 months practical experience with the methods,
  techniques, practices, aids, equipment, and tools used to perform the
  maintenance, preventive maintenance, or alterations; and
(2) Thoroughly familiar with the applicable regulations in this
  chapter and proficient in the use of the various inspection methods,
  techniques, practices, aids, equipment, and tools appropriate for the
  work being performed and approved for return to service. (c) A
  certificated repair station must ensure each person authorized to
  approve an article for return to service understands, reads, and
  writes English.

They also must meet the requirements set fourth in:

(b) Meets the eligibility requirements of §65.101.

which you can find here.
You can find the full regulations pertinent to becoming a repair station here

Do some more reading at the link you provided in these paragraphs. I think that will answer your question.
§65.80   Certificated aviation maintenance technician school students.
§65.81   General privileges and limitations.
§65.83   Recent experience requirements.
§65.85   Airframe rating; additional privileges.
§65.87   Powerplant rating; additional privileges.
§65.89   Display of certificate.
§65.91   Inspection authorization.
§65.92   Inspection authorization: Duration.
§65.93   Inspection authorization: Renewal.
§65.95   Inspection authorization: Privileges and limitations.

A certificated mechanic with an airframe rating can approve and return to service an airframe, or any related part or appliance, of an aircraft with a special airworthiness certificate in the light-sport category after performing and inspecting a major repair or major alteration for products that are not produced under an FAA approval provided the work was performed in accordance with instructions developed by the manufacturer or a person acceptable to the FAA.
A certificated mechanic with a powerplant rating can approve and return to service a powerplant or propeller, or any related part or appliance, of an aircraft with a special airworthiness certificate in the light-sport category after performing and inspecting a major repair or major alteration for products that are not produced under an FAA approval, provided the work was performed in accordance with instructions developed by the manufacturer or a person acceptable to the FAA.

and further past the A&P ratings above is IA:

The holder of an inspection authorization may—
(1) Inspect and approve for return to service any aircraft or related part or appliance (except any aircraft maintained in accordance with a continuous airworthiness program under part 121 of this chapter) after a major repair or major alteration to it in accordance with part 43 [New] of this chapter, if the work was done in accordance with technical data approved by the Administrator; and
(2) Perform an annual, or perform or supervise a progressive inspection according to §§43.13 and 43.15 of this chapter.


text:
Why were turbojets the first kind of turbo-engines, instead of turboprops?
During WW2, the first jet engines appeared en mass in combat aircraft. Britain and Germany made the first ones.
But why were turbojets first? Why not turboprops, which are easier since the turbo-machinery is smaller for a given thrust. Turboprops have propellers so it would've been a much easier evolution too. If I'm not mistaken, turboprops can fly higher and faster much more easily than piston-props, so it would've conferred an advantage.
So why were turbojets first? Did the concept of a propeller attached to a turboshaft just not cross the minds of engineers during the war? Was it tunnel vision onto the much more powerful turbojets?

Or maybe the gearbox was the component creating the problem, this was a huge power to transfer.

The Merlin had a reduction gearbox, for instance the Merlin XII Merlin fitted with 0.477:1 reduction gear installed in some Spitfire II's with three-bladed Rotol constant-speed propeller. Rated at 1,150-horsepower (857 kW) at 3,000 rpm at 14,000 feet.[19])

The development of turboprops and jets were going on more or less concurrently before and during the initial stages of WWII -their first test runs were within years of each other. You have to understand that there was a war going on- any (significantly) better aircraft engine would be the difference between life and death; and that would be the one that would get the resources. The turboprop was not to be that engine. During development, the turbojets showed improvements and promise that the turboprops never did.
Turboprops have propeller but the engine was completely different- instead of developing another type of engine (which will certainly be less powerful compared to the present ones) and making it to run the same propeller, it is easy to see why the concept of an engine without propeller is more appealing- for similar configuration, you have to show a significant improvement downright, while the new idea can survive (atleast initially) on promise. Add to this the fact that the piston engines reached their peak during WWII in performance and power, it would have been a tall order for the turboprops to better them.
Generally speaking, while turboprops of today may be faster than the piston engined aircraft, it was not necessarily true till the Soviets developed the Tu-95. In case of jet engines however, the difference was substantial. One of the initial problems in Me-262 was that it was too fast for prevailing tactics. During their introduction, the jets didn't offer any meaningful increase in flight ceiling.
For a turbprop, you need three parts- a turbojet, a gearbox and a propeller. A turbojet eliminates two of these components and makes the the engine simpler to construct (even compared to some piston engines), an important consideration in wartime.
At the time of the war, the primary thrust of engine development of improving the performance of combat aircraft- and the jet engines brought a paradigm shift in this regard by increasing speed, which the turboprop was never able to do. Even today, the turboprops are not popular for their power, but for their endurance, and their application in combat role is rather limited; in a situation like WWII, where the combat aircraft were paramount, their ascent would have been inconceivable. It was not tunnel vision, but facing the reality of appropriating the limited resources for surviving today and winning tomorrow.

Propeller planes already existed, and turboprops were an answer to a question that was not asked in times of war. The main driving factor behind development of the turbojet was to provide great climbing power and high airspeed for intercepting enemy bombers, which the turboprop would not have been able to do. Fuel economy and engine life were of secondary importance, the Junker-Jumo 109 of the Me-262 lasted only 10 flight hours!
The piston propeller engines were very sophisticated towards the end of WWII, the super-charged Merlin piston engine powering the Spitfire developed over 2000 hp.

But the Me-262 and the He-178 were way faster than the Mustangs & Spitfires that escorted the bombers, and could only be shot down when lined up for landing. From the wiki:

Too fast to catch for the escorting Allied fighters, the Me 262s were almost impossible to head off. [Note 7] As a result, Me 262 pilots were relatively safe from the Allied fighters, as long as they did not allow themselves to get drawn into low-speed turning contests and saved their maneuvering for higher speeds.

Turboprops would not have resulted in such an advantage, since the propeller is the limiting factor for airspeed. The lighter weight of a turbine would have been beneficial: the Merlin weighed 600 kg, and for instance the PW123 turboprop delivers 2,380 hp and weighs 450 kg - but only beneficial in the lower speed turning performance which the jet fighters learned to avoid.


text:
What is a high speed stall?
I'm told that planes can actually stall when the airflow over the wing goes past Mach 1?  Why does this happen and how do you design an aircraft to avoid it?

stall happens on high angle of attack at any speed, you avoid it by not pitching up or down in supersonic flight

I believe what you are asking about is the coffin corner.  Is this what you are interested in?  http://en.wikipedia.org/wiki/Coffin_corner_%28aviation%29

@ratchetfreak: The supersonic flow separation occurs at any angle of attack.

@JerryKur: Existence of coffin corner is a consequence of supersonic flow separation, but supersonic flow separation is not limited to it. It occurs on supersonic planes as well.

A high speed stall is something that happens close to, but still below, Mach 1. It involves supersonic flow over the wing, but the whole airplane still travels at subsonic speed.

Ah, I'll edit the question @PeterKämpf.  I must have misunderstood your comment on my other one ;).

When the velocity of the airflow locally exceeds speed of sound above the wing a shock wave forms and the flow detaches beyond this shock wave.

Similarly to stall, the supersonic separation of flow removes the component of lift produced by decrease of pressure on the upper surface of the wing and so the effects are similar.
I causes reduction of lift and because centre of pressure is about quarter chord on the upper surface, but midchord on the lower, it causes a significant pitch-down moment, which might be impossible to recover even if the post-stall lift is otherwise sufficient to keep the aircraft flying straight. This effect is often called Mach tuck. Supersonic planes often have all-moving elevators to have sufficient control authority to compensate for it.
A difference from normal stall is that after supersonic flow separation the lift remains proportional to angle of attack and so the aircraft continues to behave more or less normally except for the change in trim.
Mach tuck may occur as low as Mach 0.7 depending on aircraft design, because the air moves faster over the wing. It can be delayed by using swept wings, because the shock waves only form when the air velocity component perpendicular to the wing exceeds speed of sound.
Supersonic aircraft eventually encounter supersonic flow separation, but the lower surface lift is sufficient to balance the aircraft weight at that speed and altitude and the aircraft can continue flying with the upper surface flow separated.
That's not the case with most subsonic aircraft, which for efficiency reasons tend to cruise at altitudes where they have very small margin to stall. As the stall speed increases with altitude while speed of sound slightly decreases with the lower temperature there, the stall speed will eventually equal critical mach number, which creates the coffin corner and absolute ceiling for the aircraft. For most civilian transport aircraft the range between stall speed and critical mach number (where drag increases and supersonic flow separation would begin) reduces considerably at cruise altitude, but they usually don't have enough engine power to reach the actual coffin corner.

Mach tuck is the phenomenon of trim change due to the rearward march of the center of lift. It can happen in a vertical dive, even when the wing does not need to produce any lift. A stall is always when the desired lift cannot be produced and the aircraft leaves the linear range of the lift curve slope.

@PeterKämpf: I reformulated the answer to avoid using the term "stall" for anything related to supersonic flow separation.

@PeterKämpf A minor point, but mach tuck has nothing to do with trim.  It refers to the nose-down pitching moment caused by the aft shift of the center of lift, which gets progressively worse because as the nose drops the speed increases even more, which leads to the C/L shifting further aft, etc, etc.  Trim isn't involved (other than for aircraft with Mach Trim systems which are designed to prevent mach tuck from happening).

@Lnafziger: I should have selected a different word. For me, trim is the balance of forces around an axis. For you, trim is the action of changing them, or maybe even the control for doing this. My understanding of the word comes from Naval English, and it seems it does not apply to aviation.

@PeterKämpf: Ahh, I understand.  To a pilot, trim has a very specific meaning, which is to use the control which moves the trim tab/movable control surface other than with the yoke, which changes the trim of the aircraft.  Since that is an action that we perform (we trim the aircraft) it is How most likes will understand that comment.

@Lnafziger: I am a pilot, too. But also an engineer, quite like you. For a pilot, trimming means to set control forces to zero. My formulation of "trim change" meant to describe this, like you change the trim of a boat to adapt to changing winds. How would you propose to call the balance of forces around the primary axes?

@PeterKämpf Good question.  I'm not an aeronautical engineer, so I didn't realize that the term was used for the balance of forces, but it makes perfect sense now that you have pointed it out.  I'm not aware of another term for it, and have typically seen a phrase very similar to what you just used (either the forces are in balance or they are imbalanced).

Aircraft stall when the wing cannot produce enough lift to sustain flight. This can happen for two reasons:

Flow separation due to high angle of attack. The lift curve slope, which is positive and linear at low angles of attack, becomes negative, such that an increase of the angle of attack results in lower lift. This is caused by viscosity effects.
Flow separation and heady buffeting due to strong shocks on the wing close to Mach 1. This is caused by Mach effects. In many cases lift loss is not yet critical but buffeting will become uncomfortably severe and dangerous to structural integrity.

The first is a low speed stall, but it can happen at any speed. High speed stalls are the second variety. They can be provoked by

flying faster at the same attitude, normally in very thin air and at speeds around Mach 0.8.
Or they can be provoked by demanding more lift at the same speed, e.g. by initiating a turn.
A third way is by climbing, such that the air gets thinner and colder, and the wing needs a higher angle of attack to produce the same lift as before.

In all cases, the initial flow over the wing was locally mildly supersonic and produced a weak shock. Either by increasing speed (more precisely: The flight Mach number) or angle of attack, the shock becomes stronger and can cause flow separation, such that the wing produces less lift than before. The aircraft stalls. In case of the U-2, the tail would still work, only the wing would produce less lift, so the plane pitches down and accelerates. Acceleration makes things worse, because now the shocks grow even stronger. Now the pilot is locked into a dive which he cannot end. A lot of fun when a few kilometers below the pilot sees MiG-17s, performing pull-ups from high speed to get close enough to open fire.
Thankfully, the drag increase due to the stronger shocks limits the speed increase, and after dropping for maybe 2km, the air becomes dense enough for the U-2 pilot to successfully stop the dive and start the climb back to the safer, higher altitude.
How to avoid this? Generally, this is impossible to avoid altogether, it can only be mitigated. Wing sweep is the most effective way to limit lift loss due to local shocks, but even a highly swept delta wing will have a reduction of its maximum lift coefficient around Mach 1. An optimized airfoil shape helps to push the limit up, but when this new limit is exceeded, the lift loss will be heavier. In the end, most modern aircraft resort to limiting the flight envelope and using an electronic FCS to ensure that the envelope is not exceeded.

Good answer (+1!) but is this actually called a "high speed stall"?

@Lnafziger: If the dynamic pressure is not sufficient, it is a low speed stall. No matter if you pull 8g, the best way out is a speed increase. A high speed stall is when increasing Mach number effects reduce lift below what is required, either by accelerating or by pulling gs with sufficient dynamic pressure.

That doesn't actually answer my question....

@Lnafziger: Yes, this is called a high speed stall. At least to my understanding. Does this answer your question?

According to "how", stall does not imply the lift not sufficient to sustain flight, only that the wing is in the flying under conditions where the lift no longer increases with angle of attack. Not sure whether there is any "official" definition though.

@PeterKämpf Yes, it does (thank you), however I have never heard this term actually used.  Do you happen to have a reference available to support the use of the term?

@Lnafziger: Unfortunately, no. My use is from reasoning, because stalls due to viscosity effects are the same, regardless of load factor: The aircraft has too little speed. Stalls from compressibility effects are in a different category, so they deserve a different name. I have met several people who referred to a viscosity-related stall at more than 1g as a high-speed stall, but they were all rather ignorant of what was going on. I also do not know of any "official" authority for naming these phenomena, so I rely on my own understanding of aerodynamics.

Well, that makes sense from a reasoning point of view, however telling someone that the definition of a term that they made up is something, when it isn't a recognized term in the industry, may be a bit...  misleading.  I understand that the OP didn't quite understand what they were asking, but they did make it pretty clear from the description.  In any case, I only object to you presenting this as an actual definition to a made-up term. Adding something as simple as "what I call a high-speed stall..." would help to clarify for people in the future that it isn't "fact".

@Lnafziger: With other aerodynamicists I never had a problem to agree on this definition, and in University I never heard the term being used for a regular stall at higher load factor. That meaning I encountered later, with private pilots who used it to embellish their stories of their exploits. With a FAR-23 propeller airplane one will never encounter a high speed stall, but the term sounds so much cooler. But again, I have no source for an authoritative definition. And I certainly did not make up the term.

Well, I didn't mean to say that you just made it up, but rather just that it wasn't a standard term with this meaning. At any rate, incorporating that little bit of background can help future visitors understand that this term may be a bit of short hand it slang and not really an official term that is taught. The way that your answer is written now makes it sound like this is the actual definition of the term. Just a suggestion though, and I'll quit bugging you now!

High speed stall is associated with buffeting due to the aforementioned flow separation. Generally the lift is well-maintained even after flow separation. But the buffeting becomes so strong that normal piloting is untenable (deterrent buffet).

@Jimmy: Yes, thanks for mentioning buffeting which in many cases will come before lift loss. I added that point to the answer.

This can happen at high altitude. At high altitudes, the air is less dense, (there are fewer molecules per cubic inch). Lift is generated by the force of air molecules hitting the surface of the airfoil. So at altitude, it takes a higher angle of attack on the airfoil to generate enough lift to hold the aircraft in level flight. Therefore, even if supersonic, there may not be enough molecules hitting the wing to generate sufficient lift at the stall angle of attack. ergo, the aircraft, in attempting to maintain level flight, increases AOA past the stall AOA.
Even then, of course, all the pilot has to do to recover from the stall is reduce AOA below the stall AOA to resume controlled flight. The aircraft will of course begin to descend and he will have to deal with a dive recovery once he reaches an altitude where the air is dense enough to support 1 G level flight.


text:
What is the aspect ratio of a wing?
I've been trying to get my head around wing design and have been following a few YouTube videos. When people talk about aspect ratio (AR) on a wing and give a value (Piper PA28 having an AR of 5.7) what is the 5.7? Is it wingspan length? Square metre of coverage? Or something else? I'm confused.

Related: http://aviation.stackexchange.com/questions/716/what-dictates-the-aspect-ratio-of-an-aircrafts-wing?rq=1

The Aspect Ratio ($AR$) is defined as the ratio (hence the name) between the wingspan squared $b^2$ and the area of the wing $S$.
Since both $b^2$ and $S$ are measured with a length squared (e.g., $m^2$), $AR$ is an dimensionless quantity.
To answer the follow-on question of TomMcW in the comments, $b$ is the distance between the wingtips, no matter the shape of the wing.

How is wingspan figured on a swept wing? Is it perpendicular distance between wingtip and fuselage or length of leading edge?


text:
Why has Boeing used mini winglets on the 737-200?
I found mini winglets just in B 737-200. What's the differences between mini winglets and the blended ones which Boeing uses in more recent versions?
(picture source)

When originally Richard Whitcomb of NASA developed the winglets, he developed the split winglets with one above and one below as shows in image below:

The one above starts at 0.4x chord from the leading edge. For a test program on KC-135 tanker, which I suppose was first application of the winglets, they used the type of winglets shown in the question:

Conceptually there is no difference in the "mini-winglets" and the "Blended winglets" While I think almost all the wingtip shapes are more or less of same efficacy, I can think of couple of reasons for variations seen among various aircrafts - One is patents taken for some shapes would force other OEMs to come up with different shapes. Second is weight of the wingtip vis-à-vis the reduction in fuel burn. Typically winglets reduce fuel burn by ~3-8% but in some cases, the increased weight can off-set majority of the fuel burn or even prove to be more than compensating for it, especially on shorter hauls. One may be able to make a lighter blended winglet using co-bonded, co-cured composite structure as compared to the short winglet mentioned in the picture which with say bolted joints might end up heavier. I think it would be difficult to say make any generalised comment on this.

Boeing 737-200s are comparatively old and the winglets developed for them are not by Boeing, but by a third party, as noted by b737.org.uk:

... 737-200..., fitted with mini-winglets. This is part of the Quiet Wing Corp flap modification kit which gained its FAA certification in 2005.

The other types of winglets in B737s- blended wing winglets in 737-800s, the AT winglets in 737 MAX and Split Scimitar in NG 737 (through there is some overlap here) were all developed by Boeing (with others) and are available from production units and also as retrofit.

While this is a fair remark, I feel it doesn't answer the spirit of the question - why are these winglets so small ?

Adding winglets to an existing design puts stresses on the existing spar ends that they weren't designed for. Fortunately, they're stronger than may be necessary at the ends due to mechanical implementation requirements, allowing some additional load. But a "full size" blended winglet should be designed in from the start to ensure sufficient strength, minimal weight and avoidance of aeroelasticity (flutter).

Yet there are extensive full size winglet upgrade programs for existing aircraft. And if you start with the design there may be much better options than winglets. They are beneficial for existing wings because they can deliver a slight wing extension while their weight relieves the root bending moment, and yes the wing/winglet combination needs to be tested for adequate strength and aeroelastical stiffness.

One story I've heard is that additional load due to winglets was not a problem, as the wings were slightly over-dimensioned initially due to larger uncertainties in the stress calculations. Over time, stress calculations methods improved and some strength margin was found which could be used to handle the additional load of the winglets.

The mini winglets require fewer modifications on the wings, and the -200 being much older does not warrant large investments in fimproved fuel economy.

The pic above, from this Boeing article on the NG winglets, shows the work required for winglet retrofit, based on stress analysis at ultimate loads in various aircraft states. The winglet shape basically extends the wingspan (but the weight helps to reduce the wing root bending moment), and creates aerodynamic lift and thrust as presented here.
The large winglet on the 737-800 is the result of a large and costly engineering effort, very particularly matched with the wing properties and the payload/range mission of the aeroplane. To make such a design effort for the -200 with its much higher fuel burn rate would be cost prohibitive.
Wiki Commons pic source
The small winglets on the -200 pick the low hanging fruit of fuel burn reduction by reduction of wing tip vorticity. Same as the original winglets on the A320, from before they offered the Sharklet as an improved option. The deleted answer from @FeliceGraziono is correct:

Being smaller, they impart less efficiency but also stress the wing root less.


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What is a good place to start learning about pulse jets?
I'm a student who is curious about designing his own pulse jet engine, I am wondering if there are any schematics designs, notes, and other related materials I could/should study to learn about these engines. Where can I find such things and what is a good place to start?

I would start with Wikipedia Pulsejet. In the "External links" section, there are quite a number of links to specific resources.

You could try this website on home made pulse jets looks like it has some good info. These guys are selling some plans and have info as well. Here is an interesting instructables... And here is another DIY one. You may also be able to dig up some of the work that was done with them during WWII with a bit of directed searching on some public archives (as it may be more or less public record now). Of course physics and engineering text books will always be of help but there may not be any pulse jet specific ones.
PSA: You should take extreme caution when building and testing something like this. You will presumably be burning jet fuel (or another comburent) in a home made combustion chamber which can lead to very bad things. Care should always be taken when firing up an engine (of any kinda) for the first time. Welds should be double checked, metal choice is important and please wear safety goggles!

Thanks a tonne! Had given some thought to studying the Argus engines used for The V1 bombers and Ford Cruise missiles, but this helps alot!

Be careful with the V1 bomb (AS014 Engine) those pulse jets were made to last for only one flight and a lot of corners were cut from a materials standpoint so i would not necessarily try to mimic it, the physics however will always apply.

On the crazy (also known as "don't try this at home") end of the scale, there's Colin Furze on youtube.

If you look for performance data on the As-014 pulse jet, NACA did some thorough testing of it in 1945. The resulting report E5J02 can be found here. It contains information about thrust, fuel flow, pressures and information about the test setup and procedure.
This Russian site has lots of drawings and detail pictures.


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In what ways is color blindness an issue in modern jet airliners?
What are the specific problems in modern jet airliner cockpits or in piloting operations that would prevent a person with any type or severity of color blindness from completing his/her job to the same degree as a pilot without color blindness?
As a follow up, are modern airliners designed to allow for pilots who have any type or severity of color blindness in case regulations change to allow them to work?

related: http://aviation.stackexchange.com/questions/16098

Thanks, I know this is related but it doesn't answer my more specific question so I continued with the post.

Now that I think about it, I'm surprised that nobody in the US has attempted to challenge the aircraft/airport/navigation chart lighting/coloring schemes under the ADA. I'm not, in any way saying I would approve of or support such a challenge, just that in our litigious society, it's surprising nobody's tried.

@FreeMan I wouldn't be surprised if someone has tried to challenge it, but, given all of the problems it would cause, any such challenge would be very unlikely to succeed. Furthermore, federal law itself isn't really subject to ADA, since Congress can just override the ADA at will (or delegate authority to do so to executive agencies.)

Another related: Do pilots who have color deficient eyesight see light gun signals differently?

@FreeMan The ADA requires reasonable accommodations. Changing ICAO standard lighting colors all over the world is not "reasonable" by any sane standard, nor is making the US a Special Snowflake. By extension allowing a pilot to fly when they can't perceive the standard lighting colors is inherently unsafe & therefore not "reasonable" either. I know a lot of lawyers, but none crazy or drunk enough to take that case :)

Australia had someone challenge the rule that color blind people may not be allowed to fly aircrafts. Well, they won. It was a different approach though,  they check if a pilot can use the helps provided by (e.g.) the PAPI lights. Not  if they can see what color they are, but really only if the pilot can use them. The US has something similar, but not as easy to pass, where you get a letter of evidence if you can identify 12 lights from the tower and read a sectional chart. For a class 2 and 1 medical you also have to take a flight and proof that you can identify good emergency landing spots.

@Maverick283 -- this is very good answer material!

There are lots of problems.

Sight is just generally important when flying. Even in airliners, most approaches are still flown manually and often visually. Any significant degradation of sight is going to be a problem when flying any type of airplane.
Red/Green/White light signals are used by air traffic control in the event of a radio failure. The inability to differentiate red from green in this case would be an enormous safety hazard (e.g. did tower just clear me to land or tell me to keep circling?)
PAPI/VASI lights use colors to indicate whether you're above, below, or on the proper glide slope for a runway. While it's possible to land without them, it's a significant reduction in safety.
Lights at airports are color-coded to indicate whether a particular road is a runway or taxiway, how close you are to the end of the runway, that a runway is in use, etc. Landing on a taxiway is usually a bad idea, so it's best if you can tell the difference.
The navigation lights on airplanes themselves are color-coded. The right wingtip has a green light, the left wingtip has a red light, and the back has a white light. This lets other pilots easily determine which way you're going and, in case of conflict, who should yield (seeing a red light on another aircraft means that they're to your right and, therefore, you should yield to [i.e. divert around] them.)
Colors are used on aviation charts to indicate all kinds of things, including class of airspace, type of special-use airspace, whether a field is controlled or not, type of route, elevation of terrain, where you should expect to see city lights at night, etc. You need to be able to distinguish these things quickly and accurately, even in low-light conditions.
Some cockpit instruments use colors to indicate various things. These include color bands on airspeed indicators, color of annunciator lights or their on-screen equivalents (i.e. red for warning, amber for caution, green for gear down, etc.,) echo intensity on a weather radar display, etc.

On point 1 - that's more to do with visual acuity, rather than color blindness. There are plenty of pilots that use corrective aids (glasses, etc). You can have 20/20 vision and still be color blind.

Most of your other points make sense of course, I'm not arguing that color blind people should be allowed to fly! I just want to know where the problems are. So it seems even if cockpits were designed to be 'color blind friendly', most of the problems areas lie in lighting outside of the cockpit.

@PaulRedmond Regarding the first point, I meant just any sort of degradation of eyesight, whether that be a problem with acuity, astigmatism, color blindness, cataracts, etc. All of these are unhelpful when trying to pick out a runway from its background (or another airplane from the background,) for example. Picking out a runway from its background can actually be harder than you might think.

The big one here is in #2, #3, and #5: The ability to differentiate three colors (Red, Green, and White) is critical to the safe performance of a pilot's duties. There are a few other colors that show up (yellow in light gun signals, blue for taxiway lights), but red/green colorblindness is the usual situation.

I will mark this as the correct answer as you have provided a number of points (2, 3 and 5 in particular) that answer the question.

@voretaq7 - Blue-yellow colorblindness is the other major partial colorblindness, and that could cause trouble distinguishing certain rotating beacons as well as telling the difference between the yellow-white runway and blue taxiway lights. No matter how you slice it, if you can't distinguish any particular primary or secondary color from another, you will be in trouble in some critical situation aboard an aircraft.

Unfortunately most non color blind people don't know what it is like to be colorblind. How would they? But it's those people that make the rules, thinking "Oh my God, they don't see colors we can not allow them in airplanes!". I'm colorblind, but when doing my PPL I could see and "read" the PAPI lights about 5nm earlier then my flight instructor, because my eye sight was better and color blind people have other means of determining color differences. So I personally thing it is wrong to exclude all color blind from flying, which in the US is not really a problem, but ICAO standards do.

@Maverick283 Ok, now I'm curious what you were doing on final more than 5 nm out while doing your PPL... As far as colorblindness is concerned, there are different kinds. I do know people that definitely can't tell the difference between red and green, for example.

@reirab Well what are you gonna do when your runway lines up with your course? And yes, there are, but that doesn't mean there are others that can't! So why strictly prohibited them from flying?

I believe the answer largely lies with history. Aircraft position indicators are red/green; airports can have white and green beacons at night. Being color-blind was a disadvantage in the past. Cockpit designers use color intentionally to help get a pilot's attention. I doubt any airline wants to go to court after an accident knowing they had a color-blind pilot involved. If there was an incredible shortage of pilots they might consider arguing for relaxed standards, but they have little incentive to do so currently, as far as I can see.

I get the point of 'Why would we hire a color blind pilot when we have hire a pilot that's not color blind?', I'm more interested in the specifics of what problems a color blind pilot could encounter these days. For the aircraft position indicators, is there something to be misinterpreted? I don't think the red/white lights would be a problem to distinguish for most people with color blindness, but I could be wrong.

@PaulRedmond Yes. Red vs. green needs to be distinguished (see my answer.)

If you had to, would it be POSSIBLE to rework all aviation standards so they'd all work for colorblind pilots? Probably, yes. For any single instance, there is probably some work-around that could be applied to make information available, more-or-less adequately, to someone who is colorblind. Maybe not AS effective as using a full color solution for those with normal color vision, but more-or-less adequate.  But the question is, why WOULD you do this?  Could the costs ever be justified by any benefits gained? And could the less effective displays (for everybody else) be justified? Doubtful!


text:
In Airbus flight control laws what does it mean that "control sensitivity depends on airspeed"?
Direct Flight Law:
What does the following characteristics exactly mean regarding sidestick input at a very low airspeed (let us assume near stall speed).
'control sensitivity depends on airspeed'
Btw. is direct flight law only achievable with gear down and both FACs off?

Remaining in direct law and with gear down at relatively low altitude could against official documentation any protections kick in automatically? Could the plane assume landing mode and enable alpha protection overwriting the pilots inputs?

If you want to ask more questions, ask more questions, don't add them as comments.

You have posted four questions as one.    Jan has answered the main question. I suggest you post the others separately and consider identifying the aircraft model of interest.

In aircraft with mechanical controls, the position of the control column corresponds to position of the control surfaces and the force on the control column corresponds to the force on the control surfaces. At higher speeds, the same deflection generates larger force and correspondingly stronger response from the aircraft.
However in Airbus the forces are not carried and the side-stick is spring-loaded. In normal law the side-stick deflection commands specific wing loading (which mostly corresponds to pitch rate) in pitch and roll rate in roll. So you get the same rate or pitch/roll with the same deflection independent of speed.
But in direct law, the position of the side-stick corresponds to the position of the control surfaces. Because the same deflection causes larger aerodynamic forces, it leads to higher pitch/roll rate at higher speed. Significantly higher, because the control forces grow with square of speed.

So does this mean that at the transition to Direct Law there will be somewhat of a step change in how the control surfaces respond to the stick? How does Airbus handle this transition? The boundary condition does not seem to match from the two sides of approach. e.g. A the same stick deflection would suddenly correspond to a grossly different control surface deflection?

@curious_cat when transitioning between laws, there is period where the signals are blended. In the beginning the original law provides 100% of the control signal, but over time the new law will gradually get a stronger weight until it is at 100%. The switch over period varies between laws.

@DeltaLima Ahh! Very smart. Thanks.

@DeltaLima what duration are the periods of transition?

@Pugz That would be a good question to ask on this site. That way the information given in the answers is much easier to find using the search functions.

As to the second part of your question, AFAIK the aircraft will only go into Direct Law or Alternate Law in the case of a malfunction, either in the physical control systems, sensors or computer systems. In the case of Direct Law all flight envelope protections are lost. Since the controls are not connected with cables in a FBW system you still need the FAC's since they are actually making your control inputs happen.

Direct law:
When the data for the C* law is no longer valid, or upon landing gear extension when the alternate law is active, the direct law is activated in pitch. The side stick directly controls the elevator position. Therefore the side stick position is no longer generates a load factor demand as a function of the position of the side stick and inertial feedbacks.
The THS is mechanically controlled and no flight envelope protections are included.
In lateral control, the side stick movements directly control the position of the ailerons and the spoiler 4 and 5.


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How would the Airbus A320 PFD respond to failure of all pitot and AOA sensors?
In theory, if all 3 pitot tubes and all 3 AOA-sensors would fail simultaneously for a period of time, could the PFD still deliver correct roll angle and horizontal displays? How would the plane calculate the actual airspeed?

The primary flight display¹:

is a single display, but it shows five separate instruments:

On the left, the speed tape. This shows the value from the pitot probe.
In the middle the attitude indicator. This shows attitude from gyroscopes. All gyroscopes are grouped in the “inertial reference unit”.
On the right the big tape is altitude. This shows the value from the static port.
The smaller tape at very right is vertical speed. It is normally taken from the inertial navigation system, which is part of the inertial reference unit, but can fall back to the static port.
On the bottom is compass. This shows the value from the inertial reference unit, which has drift automatically compensated by reference to magnetometer.

The angle of attack is not displayed anywhere. It is only used for stall warning, as part of the alpha-protection logic and for checking the weight set in FMGC.
For all the instruments, each instrument shows value from one input. The three pitot probes, static ports and inertial reference units each feed one of the three displays in the cockpit: captain's PFD, copilot's PFD and the standby instrument (which in the modern version looks like smaller PFD and is placed between captain's (left) displays and the centre display.
When any of the input fails, the computer will generate a warning (accompanied with the annoying cricket sound) stating that the values do not match, but it leaves it up to the pilots to determine which one is wrong. The pilots can then switch the displays to show the one they determined as trustworthy.
So:

If all pitot probes failed, the speed would show nonsensical values, but the attitude, altitude and heading would remain operational. The system would fall back to alternate law. There is an unreliable airspeed procedure where the aircraft is kept in safe speed range by reference to attitude and vertical speed. Failure to execute that procedure properly was an important part in the accident sequence of Air France flight 447.
If the angle-of-attack vanes fail, the stall warning and alpha floor protection will not work, but all displays would work. However, the system may actually not notice that they failed as evidenced by the accident of XL Airways Germany flight 888T. This was a test flight where they intentionally tried to activate the alpha-floor and it did not activate without any prior indication because the AoA-vanes froze due to improper wash.

¹ The image shown clearly is supposed to be Airbus PFD, but some elements like the protection indicators and the Mach number are missing. I just didn't quickly find better as this one is very readable.

AoA (Angle of Attack) vanes, as the name says, only measure the angle of attack, i.e. the angle between the velocity vector and the x-axis of the aircraft reference system.
This is used neither for roll angle measurements (the inertial units do that) nor for airspeed measurements (it is taken care of by the pitot tubes).
So if all 3 AoA sensors should fail simultaneously, the PDF could still display the artificial horizon and the airspeed correctly.

But what if theoretically all 6 probes would fail for a certain peroid of time?

Would there be any way the flight computers could calculate the actuall airspeed?

make enough sensors fail and it will not be able, but that's a different question.


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What is the minimum distance between two civil airplanes parked in a hangar?
When parking two airplanes in the hangar, what is minimum safe distance between them?

Welcome to aviation.SE! Are you asking about airliners, or any civil aircraft? If you look in a shared hangar at a GA airport, you'll often see aircraft packed in with only a few centimeters between them (and between them and the walls).

Minimum safe distance == "not touching"

Yaa , for airlines. Is there any besic rules to them on hanger.

@casey you should have linked this video with your "not touching" :D

Contact between aircraft in large hangars used to be quite common. We used to refer to minor scrapes as "hangar rash."

As a student pilot, when I completed my 3-stop, long solo flight, I returned to the FBO after hours.  The hardest part of the entire flight was parking my plane back in its spot, by myself, with less than 2 inches of room on each wing.  If it fits, it fits.

If you look at some civil-aircraft hangars, you will see that the planes are almost touching each other, as you can see here:

The planes are so close to each other to fit a maximum amount of planes into a hangar.
There are different types of hangars. Some are built just to store aircraft when they aren't used, some are built for aircraft-maintenance-operations and can even contain robots. Hangars can be simple buildings with a big and open space or they can be very complex facilities.
If you want more infos about aircraft hangars take a look here.
You can find the minimum separation distances between aircraft and minimum distances between the aircraft and other obstructions here
Take a look at this table from this document (page 30):

As you can see, the minimum distance between the wings from two parked aircraft is not always the same.

Dimensions vary for different models and configurations of aircraft.

If you want to find out the min. distance of an A380, you need to take a look at the documents provided by Airbus.
No, there is no basic rule. The safe distance is determined by the aircraft-type, configuration and the hangar (owner).
If the airliners are stored in the hangar, they can be pretty close to each other. However, maintenance of a big airliners takes up a lot of space:

What's a roboter?

@JeffreyBosboom I'm sorry, I misspelled "robot". " roboter" is german for "robots".

When stacking planes vertically, make at least sure an deflated tire will not lead to a contact. It is prudent to leave a little more space than "not touching".

@PeterKämpf By "stacking planes vertically" you mean placing them on top of each other? How can we achieve that without physical contact?

@jklingler e.g. by placing a high-wing aircraft close to a low-wing aircraft so that the wings of the former overhang the latter, or a small aircraft under the wing of a much larger one.

@DanHulme I see, thank you :)

@DanHulme and here I was thinking hanging them from the ceiling ;)

The AF Block dimensions cover the space needed on the ramp, not in the hangar. Block space is sufficient to allow aircraft to taxi in and out of position as well as provide for maintenance/service vehicles. Inside AF hangars, I've seen both less space where it's primarily just parking to much more space where there's heavy maintenance.

"Don't hit the other planes. Don't hit the walls. Don't hit anything else in the hangar." - that's the minimum safe distance.
There may be other requirements (e.g. by fire code: the number of aircraft stored in a hangar may be restricted by the amount of fuel each aircraft holds, and by the hangar's fire suppression capabilities).
Similarly in a hangar where active maintenance is happening the space between each aircraft may be larger to allow maintenance personnel access to each aircraft.

Sometimes the wings will overlap, with a high wing plane's wing over those of a low wing plane.

and with gliders, you'll see them all with one wing on the ground and one wing in the air, all tilted the same way, with sandbags on the low wing.